The continuous growth of bandwidth requirements in optical-based communications systems has resulted in a large demand for systems able to operate over a large spectral window partly outside the amplification band provided by Erbium-doped fiber amplifiers. Erbium-doped fiber amplifiers effectively operate over a limited wavelength band. Depending on amplifier configuration and fiber composition, Erbium-doped fiber can be used for amplification of optical signal defined over the telecommunications C-band and L-band i.e. respectively from approximately 1528 nm to 1568 nm and further up to 1620 nm. But, at least several different erbium-doped fiber amplification configurations would be required to cover this entire range which implies a high cost. And more severe restrictions will come from the use of Erbium-doped fiber amplifiers due to a to inhomogeneous gain spectrum with relatively high noise figure. Other rare earth-doped fiber amplifiers have been used for amplification outside the erbium wavelength band. But they exhibit very low efficiency as well as other technical problems associated with each particular kind of dopant when compared to Erbium-doped amplifiers.
Accordingly, other amplifier configurations have been develop to amplify wavelength band ranges greater than can be amplified with singular rare earth-doped amplifiers. An example which have attracted much attention comprises a Raman fiber amplifier as they can be used to extend the reach of long haul Dense Wavelength Division Multiplexed (DWDM) communication systems. Such amplifier converts laser radiation from a pump laser into another wavelength range through stimulated Raman scattering. More specifically, Raman scattering operates on the principle of Stokes light generation, which is downshifted from the optical pump frequency by an energy determined by vibrational oscillation modes in the atomic structure of the fiber. In other words, Raman gain results from the interaction of intense light with optical phonons in the glass, and the Raman effects leads to a transfer of power from one optical beam, or the pump, to another optical beam, or the signal. During a Raman scattering effect, the signal is downshifted in frequency i.e. upshifted in wavelength by an amount determined by the vibrational modes of the glass or the medium.
In operation, a pump laser is used to conduct pump radiation through a Raman medium. Signal radiation which propagates co-linearly with the pump will be amplified by stimulated Raman scattering, whereby a pump photon is stimulated to emit an optical phonon and also a photon at the same energy and phase as the signal photon. A counter propagation of signal radiation versus pump radiation is also conceivable. The wavelength range over which amplification occurs is referenced to the wavelength of the optical pump and the bandwidth is determined by the phonon spectra of the Raman medium. A direct consequence of this is that amplification can be realized at any wavelength in an optical fiber by correct choice of the wavelength of the optical pump.
One of the problems generally associated with Raman amplifiers is the requirement of a relatively large pumping power. A significant advantage, however, of Raman amplifiers is the low noise figure associated therewith close to the quantum limit of 3 dB.
In WO02/056510 is described an optical signal amplifier that includes a Raman fiber amplifier with a semiconductor optical amplifier. It makes use of the low noise figure typically associated with Raman amplifiers, the significant gain at the optical signal wavelength typically associated with semiconductor optical amplifiers and the residual pump power from a Raman amplifier to increase the saturation output power of semiconductor optical amplifier.
In “Raman amplification using high-power incoherent semiconductor pump sources” from D. Vakhshoori et al., PD47-1, OFC-2003, is described a distributed Raman amplifier using high-power and spectrally incoherent semiconductor pump sources. More than 250 mW of broadband Amplified Spontaneous Emission (ASE) has been efficiently generated from a single spatial mode semiconductor source. This was achieved through coupling of a low-power seed optical signal from a semiconductor ASE source into a long-cavity semiconductor amplifier waveguide which was optimized in design for center wavelength power amplification. Two seed sources and four power optical amplifiers devices were multiplexed in power and wavelength within the some butterfly package. This paper shows clearly that the achievement to amplify wavelength over a big band range at least covering the telecommunications C-band implies the use of several semiconductors pumps emitting at different wavelengths and multiplexed in polarization. This is irremediably connected to high cost due to the requirement to pay for several pumps for covering at least the C-band gain spectrum. Furthermore, discrete wavelengths means resulting gain excursion in the Raman gain spectrum.